Upgrading to Elements 2018 should I delete my old version first ?

Nope. You can have as many versions of the program on your computer as you'd like. They don't interfere with each other.

Also, if you're working on any projects in your current version, you should finish it in that version rather than opening it in Premiere Elements 2018. Opening an older version project in a newer version of the program can cause corruptions.